Organic light emitting devices are very sensitive to moisture. For this reason organic light emitting devices generally comprise a TFE, which may comprise a single inorganic layer or a combination of inorganic and/or organic layers forming a multilayer stack. The TFE generally provides a good protection against moisture. However, the quality of the TFE barrier may be reduced due to pinholes or voids, thereby providing penetration pathways for moisture through the TFE.
U.S. Pat. No. 7,767,498 B2 discloses a system for testing the effectiveness of barrier structures. The system uses metallic calcium coupons on glass encapsulated with multilayer barrier stacks. The formation of transparent calcium oxide and hydroxide by permeation increases the transmission of visible light through the calcium coupon, which can be optically detected for determining a degree of permeability of the multilayer barrier stacks. This calcium test is technically relatively complex and takes a relatively long time.